You answered your own question ... 16 inch gun with a a rifle lenght gas system.
From the gas port to the muzzle is blank inches, much shorter than a regular rifle gas lenght barrel of 20 inches to 24 inches. As the bullet passes the gas port hole it starts its cycle to feed the bolt carrier key with gas which propels the carrier in reverse. The lenght of barrel which is between the gas port and the muzzle is shorter so the gas port hole must be larger to provide enough volume of gas to operate the bolt carrier's weight.
Adjustable weight on the carrier , adjustable gas blocks , different weights of buffers all fix a bad port size. If the port size is right the ejection of brass is at 3 O'clock and we would not have to fix anything.
SS the last part of that rambling statement was not for you but just info.